SIP-145 Emit the proper cached debt number when debt snapshots are taken.

contracts/DebtCache.sol

@@ -34,9 +34,10 @@ contract DebtCache is BaseDebtCache {
             _cachedSynthDebt[currencyKeys[i]] = value;
         }
         _cachedSynthDebt[EXCLUDED_DEBT_KEY] = excludedDebt;
-        _cachedDebt = snxCollateralDebt.floorsub(excludedDebt);
+        uint newDebt = snxCollateralDebt.floorsub(excludedDebt);
+        _cachedDebt = newDebt;
         _cacheTimestamp = block.timestamp;
-        emit DebtCacheUpdated(snxCollateralDebt);
+        emit DebtCacheUpdated(newDebt);
         emit DebtCacheSnapshotTaken(block.timestamp);
 
         // (in)validate the cache if necessary

test/contracts/DebtCache.js

@@ -1200,9 +1200,10 @@ contract('DebtCache', async accounts => {
 			});
 
 			describe('after the synths are exchanged into other synths', async () => {
+				let tx;
 				beforeEach(async () => {
 					// Swap some sETH into synthetic dollarydoos.
-					await synthetix.exchange(sETH, '5', sAUD, { from: account1 });
+					tx = await synthetix.exchange(sETH, '5', sAUD, { from: account1 });
 				});
 
 				it('non-SNX debt is unchanged', async () => {
@@ -1214,6 +1215,39 @@ contract('DebtCache', async accounts => {
 				it('currentDebt is unchanged', async () => {
 					assert.bnEqual(currentDebt, await debtCache.currentDebt());
 				});
+
+				it('cached debt is properly updated', async () => {
+					const logs = await getDecodedLogs({
+						hash: tx.tx,
+						contracts: [debtCache],
+					});
+
+					const cachedDebt = (await debtCache.cacheInfo())[0];
+					decodedEventEqual({
+						event: 'DebtCacheUpdated',
+						emittedFrom: debtCache.address,
+						args: [cachedDebt],
+						log: logs.find(({ name } = {}) => name === 'DebtCacheUpdated'),
+					});
+				});
+			});
+
+			it('is properly reflected in a snapshot', async () => {
+				const currentDebt = (await debtCache.currentDebt())[0];
+				const cachedDebt = (await debtCache.cacheInfo())[0];
+				assert.bnEqual(currentDebt, cachedDebt);
+				const tx = await debtCache.takeDebtSnapshot();
+				const logs = await getDecodedLogs({
+					hash: tx.tx,
+					contracts: [debtCache],
+				});
+
+				decodedEventEqual({
+					event: 'DebtCacheUpdated',
+					emittedFrom: debtCache.address,
+					args: [cachedDebt],
+					log: logs.find(({ name } = {}) => name === 'DebtCacheUpdated'),
+				});
 			});
 		});